Having a license is not the same as being a paying subscriber, a requirement. If you are a subscriber then you must use the exact same LOGIN in mobile that the license using desktop AutoCAD is using: it works for all of us AutoCAD subscribers that way.
If you are not a subscriber, aka you have an old AutoCAD license that you are not paying Autodesk for, then MOBILE is not free to you at all or ever. You'll need to buy a subscription for MOBILE through the app store you got it from.
